Frequently, it is desirable in the construction of radio-frequency (RF) devices, such as transmitters, power generators and test equipment, to utilize an RF source which generates RF power with a constant amplitude while the output frequency is swept over a predetermined frequency range. As the required range is frequently large and broadband power sources with constant output over the entire band are difficult to construct, level control or gain control circuits are necessary.
Conventional automatic level control or automatic gain control circuits operate with a feedback loop that controls a linear modulator connected in series with the RF source output. More particularly, the RF output is sampled and compared to a reference level which determines the desired output power level. The difference between the output sample and the reference level constitutes an error signal which is applied to an integrator. The output of the integrator, in turn, controls the modulator so that the operation of the feedback loop drives the error signal to zero and the result is a stable output level.
The same feedback arrangement can also be used to amplitude modulate the output of the generator by simply adding a modulating signal to the reference level. The feedback loop will then force the output amplitude to follow the modulating signal.
The conventional feedback arrangement can also be used to generate amplitude modulated pulses by inserting a pulse modulator in series with the linear modulator. With this connection the pulse modulator generates pulses and the linear modulator modulates the amplitude of the pulses. In this arrangement, however, it is necessary to place a switch in the aforementioned amplitude feedback loop so that the loop is not operational between pulses.
This prior art level control circuit functions well in most situations. However, it suffers from some serious limitations. More particularly, the percentage "depth" of the amplitude modulation (AM) is limited. More particularly, because of the feedback loop arrangement, the dynamic range of the feedback loop is limited to the dynamic range of the element which has the smallest dynamic range. For example, if the linear modulator used to control the output of the power generator has a greater usable dynamic range than the output sampling circuitry, the dynamic range of the modulator cannot be fully utilized for amplitude modulation. In a typical case, the output sampling and detection circuitry limits the usable dynamic range due to DC offset drift and noise. For example, the power available from an RF generator might be approximately +10 dBm. However, in a typical RF detector, the generator output amplitude cannot be sensed accurately below -10 dBm so that the total sensing range is approximately 20 dB. Consequently, the maximum possible AM depth would be 90%. An attempt to operate the feedback system with an AM depth greater than 90% would cause severe AM distortion even if a linear modulator with an 80 dB dynamic range is used.
A second limitation of the aforementioned prior art level control circuitry is that the AM bandwidth is limited. In accordance with conventional feedback control loop theory, loop bandwidth is limited by time delays in the loop components. In particular, in the configuration discussed, the loop integrator time constant must be made significantly longer than the total delay in the rest of the loop; otherwise, the system becomes unstable. Thus, even if the linear modulator is capable of operating over a larger AM bandwidth than the feedback loop, that bandwidth cannot be utilized due to the integrator time constant.
A third limitation is that pulse modulation with the conventional circuit is limited by bandwidth degradation. Since pulse modulation increases the length of time required for the feedback loop system to respond to changes in either the reference level or the modulation signal, the AM bandwidth is effectively reduced and, in particular, the degradation in bandwidth is proportional to the pulse duty cycle. For example, if pulse amplitude modulation is being performed with 10 microsecond wide pulses spaced 100 microseconds apart, the AM bandwidth of the modulation system is effectively reduced by a factor of 10. Accordingly, this latter problem can severely limit the speed with which pulse amplitude can be modulated.
One prior art solution to these limitations is to disconnect the aforementioned level control feedback loop whenever amplitude modulation is being carried out at high rates or at large depth. The circuit then operates in a "open loop" configuration without feedback control. Conventional linear modulators and drive circuits typically have sufficient linearity to minimize gain variations and, thus, these circuits can provide amplitude modulation with acceptable distortion even without the error correcting action of the feedback loop.
In order to operate a circuit in an "open loop" configuration, the output level must be manually set. To do this, the previously-described prior art feedback loop is closed and the RF output amplitude is set to the desired level. The loop integrator output is then measured. Subsequently, the integrator is disconnected and the modulator is driven directly with a reference signal which has been set to the measured integrator output. Any amplitude modulation signals are added to this reference signal.
In this manner, the aforementioned problems with depth and bandwidth limitations are avoided, however, the level-setting procedure must be repeated whenever the RF output frequency or amplitude must be changed. Even if the RF output frequency and amplitude do not change, the level-setting procedure must be performed periodically because temperature changes cause the gains of the RF generator and modulator to change. Similarly, with such an open loop circuit it is not possible to maintain a constant RF output amplitude while sweeping the output frequency. Finally, even though the level setting procedure can be automated, it still takes a significant amount of time to perform the procedure and it is often inconvenient to perform.
Accordingly, it is an object of the present invention to provide an automatic level control circuit in which the amplitude modulation bandwidth is expanded over prior art circuits.
It is another object of the present invention to provide an automatic level control circuit which has greater pulse amplitude modulation bandwidth.
It is still another object of the present invention to provide an automatic level control circuit which utilizes a feedback loop to automatically set the level while operating in both the AM mode and pulse amplitude modulation mode.
It is a further object of the present invention to provide an automatic level control circuit in which the feedback loop is designed so that the dynamic range of all the components in the feedback loop can be utilized and the percentage AM depth can be increased.
It is still a further object of the present invention to provide an automatic level control circuit which has low distortion at high AM depth while operating in the closed loop leveled output mode.
It is yet a further object of the present invention to provide an automatic level control circuit in which the output power level is easily set to a desired value.